VULCAN Berichtzusammenfassung

Plant responses to climate change

Biomass production:The experimental warming of approximately 1ºC induced up to 100% increase in the aboveground biomass growth in the northern sites, especially in the UK and the Netherlands site. Direct and indirect effects of warming such as longer growth season (leaf unfolding was advanced on average 15 days in all sites) and increased nutrient availability were likely to be particularly important in these northern sites, which tend to be temperature-limited. No effect of drought was found in the northern sites, which are not water-limited. Instead, there was no increase in total aboveground plant biomass growth in the water-stressed southern sites, as expected since warming increases water loss and temperatures are already close to the optimum for photosynthesis plant productivity in those ecosystems. The southern sites presented instead the most negative response to the drought treatment. However, it was also noticeable that a wet year such as 2002 allowed the drought plots to recover even better than the control plots. Regarding changes in plant tissue chemistry, the most outstanding result was a decrease in the tissue P concentrations with both warming and drought. The magnitude of the plant biomass response to the treatments was thus very sensitive to differences among sites (the cold-wet northern sites more sensitive to warming and the warm-dry southern sites more sensitive to drought), seasons (plant processes more sensitive to winter warming than during summer) and species.

Roots:The effect of the treatments on root length density was not found in the soil cores of 2002 containing old roots, but was observed in the in-growth cores of 2003. The RLV of the new grown roots decreased significantly. This was most clear in the upper layer but in the lower layer this trend can also be seen. The warming treatment showed the same trend: in 2003 a decreasing tendency in most countries. The treatments apparently affect the RLV, which means that the capability of exploring the soil for nutrients and water, is affected. Drought probably negatively affects this capacity through the dying-off of fine roots. For warming, the mechanism may be different: increased organic matter decomposition may improve the availability of nutrients and subsequently reduce root growth.

Community composition changes:There were different responses in phenological advances and in growth depending on species. These different responses have changed the competitive ability of species with likely community and ecosystem level consequences, some of which have already been found during this VULCAN project. For example, a decrease in seedling diversity in the warming and especially in the drought treatment was observed in a study of the seedling recruitments in the Spanish site. In the Welsh site there was an progressive loss of dominance of Empetrum nigrum which was negatively affected by winter warming whereas Calluna increased even more its dominance. In Denmark there was an interaction of warming and drought with herbivore attack which was enhanced in both treatments either because enhanced insect metabolism in warming or because enhanced larger consumption in response to decreasing nutritional value and biomass amount in drought. These changes induce a shift in the composition of the plant community towards a decrease in biodiversity. The community structural changes will finally affect the ecosystem functioning because they affect plant growth rates and therefore C uptake or litter decomposition and therefore nutrient availability.

Decrease plant coverage and dry live fuel: erosion and fire risk:The plant coverage decreased in the drought plots in the southern sites at Spain and Italy thus increasing soil erosion risk through greater area of bare soil. The decrease in plant productivity, reproduction and recruitment means a reduced input of organic matter to the soil, and reduced soil plant coverage, leading to a decrease of the water retention capacity of the soil and to an increase of erosion risk. This is a serious risk for some areas of the Mediterranean region (see map). If the soil water content becomes less, there is a decrease in the productivity of the vegetation, further decreasing the organic matter input into the soil. Also related to erosion risk, there is the fire risk which was also increased under drought since there were decreases in water input and decreases in soil water content, increases in the ratio between dead and alive tissues, and decreases in the tissue water content making more flammable the plant biomass or ¿live fuel¿. Although there were no significant changes in the VOC content under the drought treatment, the increased VOC emissions after warming may also enhance the fire risk. In fact, in the last decades there has been a greater recurrence of fires partly linked to these effects (see satellite map). Wildfires and erosion are the current most important threads for these shrublands.